Method of manufacturing grain-oriented electrical steel sheet

ABSTRACT

Hot rolling is performed on a steel with a predetermined composition containing Ti: 0.0020 mass % to 0.010 mass % and/or Cu: 0.010 mass % to 0.50 mass % to obtain a hot-rolled steel sheet. Annealing is performed on the hot-rolled steel sheet to obtain an annealed steel sheet. Cold rolling is performed on the annealed steel sheet to obtain a cold-rolled steel sheet. Decarburization annealing is performed on the cold-rolled steel sheet at a temperature of 800° C. to 950° C. to obtain a decarburization annealed steel sheet. Then, nitridation treatment is performed on the decarburization annealed steel sheet at 700° C. to 850° C. to obtain a nitrided steel sheet. Finish annealing is performed on the nitrided steel sheet.

This application is a national stage application of International Application No. PCT/JP2011/053491, filed Feb. 18, 2011, which claims priority to Japanese Application No. 2010-033921, filed Feb. 18, 2010, the content of which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a grain-oriented electrical steel sheet in which the variation in magnetic property is suppressed.

BACKGROUND ART

A grain-oriented electrical steel sheet is a steel sheet which contains Si and in which crystal grains are highly integrated in a {110}<001> orientation, and is used as a material of a wound core of a stationary induction apparatus such as a transformer. The control of the orientation of the crystal grains is conducted with catastrophic grain growth phenomenon called secondary recrystallization.

As a method of controlling the secondary recrystallization, the following two methods can be cited. In one method, heating is performed on a slab at a temperature of 1280° C. or higher to almost completely solid-solve fine precipitates called inhibitors, and thereafter hot rolling, cold rolling, annealing and so on are performed to cause the fine precipitates to precipitate during the hot rolling and the annealing. In the other method, heating is performed on a slab at a temperature of lower than 1280° C., and thereafter hot rolling, cold rolling, decarburization annealing, nitridation, finish annealing and so on are performed to cause AlN (Al, Si)N and the like to precipitate as inhibitors during the nitridation.

In recent years, it has been requested to reduce a time taken for a decarburization annealing in a manufacturing process of a grain-oriented electrical steel sheet from a point of view of reduction in CO₂ emissions. Accordingly, it has been studied to use a slab whose C content is low.

However, lowering the C content of the slab causes a remarkable variation in magnetic property (magnetic property deviation) depending on site, after the finish annealing performed with the steel being coiled.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Laid-open Patent Publication No.     03-122227 -   Patent Literature 2: Japanese Laid-open Patent Publication No.     11-323437 -   Patent Literature 3: Japanese Laid-open Patent Publication No.     06-256847 -   Patent Literature 4: Japanese National Publication of International     Patent Application No. 2001-515540 -   Patent Literature 5: Japanese Laid-open Patent Publication No.     2000-199015 -   Patent Literature 6: Japanese Laid-open Patent Publication No.     2007-254829

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a method of manufacturing a grain-oriented electrical steel sheet, capable of suppressing the variation in magnetic property.

Solution to Problem

It turned out that the above-described variation in magnetic properties after the finish annealing is remarkable when the C content is 0.06 mass % or less, and further, it is particularly noticeable when the C content is 0.048 mass % or less. Although the cause of the variation in magnetic property after the finish annealing is not exactly known, the variation is considered to occur because the crystal grains sometimes do not uniformly grow during the finish annealing even if the crystal grains seem to be uniform before the finish annealing. Further, it may be considered that the reason why the crystal grains do not uniformly grow is because, due to the low C content, a phase transformation during the hot rolling is not sufficiently performed, so that an amount of austenite transformation is small, resulting in that a hot-rolled texture becomes unstable. Specifically, it may be considered that a sufficient secondary recrystallization does not occur in a portion in which the hot-rolled texture becomes nonuniform, resulting in that sufficient magnetic properties are not obtained.

The present inventors thought, based on such knowledge, that it is possible to sufficiently cause the secondary recrystallization through forming an effective precipitate in order to make the crystal grain growth uniform during the finish annealing. Then, the present inventors repeatedly carried out an experiment of measuring the magnetic properties of the grain-oriented electrical steel sheets obtained through adding various kinds of elements to slabs. As a result, the present inventors found that addition of Ti and Cu was effective to make the secondary recrystallization uniform.

The present invention has been made based on the above-described knowledge, and a summary thereof is as follows.

(1) A method of manufacturing a grain-oriented electrical steel sheet, comprising:

performing hot rolling of a steel containing Si: 2.5 mass % to 4.0 mass %, C, 0.01 mass % to 0.060 mass %, Mn: 0.05 mass % to 0.20 mass %, acid-soluble Al: 0.020 mass % to 0.040 mass %, N, 0.002 mass % to 0.012 mass %, S: 0.001 mass % to 0.010 mass %, and P: 0.01 mass % to 0.08 mass %, further containing at least one kind selected from a group consisting of Ti: 0.0020 mass % to 0.010 mass % and Cu: 0.010 mass % to 0.50 mass %, and a balance composed of Fe and inevitable impurities, to obtain a hot-rolled steel sheet;

performing annealing on the hot-rolled steel sheet to obtain an annealed steel sheet;

performing cold rolling on the annealed steel sheet to obtain a cold-rolled steel sheet;

performing decarburization annealing on the cold-rolled steel sheet at a temperature of 800° C. to 950° C. to obtain a decarburization annealed steel sheet;

then, performing nitridation treatment on the decarburization annealed steel sheet at 700° C. to 850° C. to obtain a nitrided steel sheet; and

performing finish annealing on the nitrided steel sheet.

(2) The method of manufacturing a grain-oriented electrical steel sheet according to (1), wherein the hot rolling on the steel is performed after heating the steel to a temperature of 1250° C. or lower.

(3) The method of manufacturing a grain-oriented electrical steel sheet according to (1) or (2), wherein the steel further contains at least one kind selected from a group consisting of Cr: 0.010 mass % to 0.20 mass %, Sn: 0.010 mass % to 0.20 mass %, Sb: 0.010 mass % to 0.20 mass %, Ni: 0.010 mass % to 0.20 mass %, Se: 0.005 mass % to 0.02 mass %, Bi: 0.005 mass % to 0.02 mass %, Pb: 0.005 mass % to 0.02 mass %, B: 0.005 mass % to 0.02 mass %, V: 0.005 mass % to 0.02 mass %, Mo: 0.005 mass % to 0.02 mass %, and As: 0.005 mass % to 0.02 mass %.

(4) The method of manufacturing a grain-oriented electrical steel sheet according to any one of (1) to (3), wherein

a Ti content in the steel is 0.0020 mass % to 0.0080 mass %,

a Cu content in the steel is 0.01 mass % to 0.10 mass %, and

a relation of “20×[Ti]+[Cu]≦0.18” is established where the Ti content (mass %) in the steel is expressed as [Ti] and the Cu content (mass %) is expressed as [Cu].

(5) The method of manufacturing a grain-oriented electrical steel sheet according to (4), wherein a relation of “10×[Ti]+[Cu]≦0.07” is established.

Advantageous Effects of Invention

According to the present invention, appropriate amounts of Ti and/or Cu are contained in the steel, and decarburization annealing and nitridation treatment are performed at appropriate temperatures, thereby making it possible to suppress the variation in magnetic property.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a chart representing the relation between a Ti content and a Cu content and the magnetic flux density and the evaluation of its variation.

FIG. 2 is a flowchart illustrating a method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

As described above, the present inventors repeatedly conducted the experiments of measuring the magnetic properties of the grain-oriented electrical steel sheets obtained through adding various kinds of elements to slabs and found out that addition of Ti and Cu is effective to make the secondary recrystallization uniform.

In the experiment, silicon steel with a composition used for manufacturing a grain-oriented electrical steel sheet based on a low-temperature slab heating method in which a C content was 0.06-mass % or less was used, for example. Further, Ti and Cu were contained at various ratios into the silicon steel to produce steel ingots with various compositions. Further, the steel ingots were heated at a temperature of 1250° C. or lower and subjected to hot rolling, and then subjected to cold rolling. Further, decarburization annealing was performed after the cold rolling, and thereafter, nitridation treatment and finish annealing were performed. Then, the magnetic flux densities B8 of the obtained grain-oriented electrical steel sheets were measured and the variations in the magnetic flux densities B8 in coils after the finish annealing were checked. The magnetic flux density B8 is the magnetic flux density occurring in the grain-oriented electrical steel sheet when a magnetic field of 800 A/m at 50 Hz is applied thereto.

As a result of the experiment, it was found out that the variation in the magnetic flux density B8 in the coil after the finish annealing is remarkably reduced when the steel ingot contains 0.0020 mass % to 0.010 mass % of Ti and/or 0.010 mass % to 0.50 mass % of Cu.

An example of the results obtained through the above-described experiments is illustrated in FIG. 1. Though details of the experiments will be described later, an open circle mark in FIG. 1 indicates that the average value of the magnetic flux densities B8 of five single-plate samples was 1.90 T or more and the difference between the maximum value and the minimum value of the magnetic flux density B8 was 0.030 T or less. Further, a filled circle mark in FIG. 1 indicates that at least the average value of the magnetic flux densities B8 of the five single-plate samples was less than 1.90 T or the difference between the maximum value and the minimum value of the magnetic flux densities B8 was more than 0.030 T. It is apparent from FIG. 1 that when the steel ingot contains 0.0020 mass % to 0.010 mass % of Ti and/or 0.010 mass % to 0.50 mass % of Cu, the average value of the magnetic flux densities B8 is high and the variation in the magnetic flux densities B8 is small.

Next, a method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention will be described. FIG. 2 is a flowchart illustrating the method of manufacturing a grain-oriented electrical steel sheet according to the embodiment of the present invention.

In the present embodiment, first, a slab is produced through casting of molten steel for a grain-oriented electrical steel sheet with a predetermined composition (Step 1). The casting method therefor is not particularly limited. The molten steel contains, for example, Si: 2.5 mass % to 4.0 mass %, C, 0.01 mass % to 0.060 mass %, Mn: 0.05 mass % to 0.20 mass %, acid-soluble Al: 0.020 mass % to 0.040 mass %, N: 0.002 mass % to 0.012 mass %, S: 0.001 mass % to 0.010 mass %, and P: 0.01 mass % to 0.08 mass %. The molten steel further contains at least one kind selected from a group consisting of Ti: 0.0020 mass % to 0.010 mass % and Cu: 0.010 mass % to 0.50 mass %. In short, the molten steel contains one or both of Ti and Cu in ranges of Ti: 0.010 mass % or less and Cu: 0.50 mass % or less to satisfy at least one of Ti: 0.0020 mass % or more or Cu: 0.010 mass % or more. The balance of the molten steel may be composed of Fe and inevitable impurities. Note that the inevitable impurities may include an element(s) forming an inhibitor in the manufacturing process of the grain-oriented electrical steel sheet and remaining in the grain-oriented electrical steel sheet after purification is performed through a high-temperature annealing.

Here, reasons for numerical limitations of the composition of the above-described molten steel will be described.

Si is an element that is extremely effective to enhance the electrical resistance of the grain-oriented electrical steel sheet to reduce the eddy current loss constituting a part of the core loss. When the Si content is less than 2.5 mass %, the eddy current loss cannot be sufficiently suppressed. On the other hand, when the Si content is more than 4.0 mass %, the processability is lowered. Accordingly, the Si content is set to 2.5 mass % to 4.0 mass %.

C is an element that is effective to control the structure (primary recrystallization structure) obtained through primary recrystallization. When the C content is less than 0.01 mass %, the effect cannot be sufficiently obtained. On the other hand, when the C content is more than 0.06 mass, the time required for decarburization annealing increases, resulting in a larger exhaust amount of CO₂. Note that when the decarburization annealing is insufficient, the grain-oriented electrical steel sheet with excellent magnetic properties is less likely to be obtained. Accordingly, the C content is set to 0.01 mass % to 0.06 mass %. Further, since the variation in magnetic property after finish annealing is particularly prominent when the C content is 0.048 mass % or less in the conventional technique as described above, the embodiment is particularly effective in the case where the C content is 0.048 mass % or less.

Mn increases the specific resistance of the grain-oriented electrical steel sheet to reduce the core loss. Mn also functions to prevent occurrence of cracks in the hot rolling. When the Mn content is less than 0.05 mass %, the effects cannot be sufficiently obtained. On the other hand, when the Mn content is more than 0.20 mass %, the magnetic flux density of the grain-oriented electrical steel sheet is lowered. Accordingly, the Mn content is set to 0.05 mass % to 0.20 mass %.

Acid-soluble Al is an important element forming AlN serving as an inhibitor. When the acid-soluble Al content is less than 0.020 mass %, a sufficient amount of AlN cannot be formed, resulting in insufficient inhibitor strength. On the other hand, when the acid-soluble Al content is more than 0.040 mass %, AlN becomes coarse, resulting in a decrease in inhibitor strength. Accordingly, the acid-soluble Al content is set to 0.020 mass % to 0.040 mass %.

N is an important element forming AlN through reacting with the acid-soluble Al. Though a large amount of N does not need to be contained in the grain-oriented electrical steel sheet because nitridation treatment is performed after the cold rolling as will be described later, a great load may be required in steelmaking in order to make the N content less than 0.002 mass %. On the other hand, when the N content is more than 0.012 mass %, a hole called blister is generated in the steel sheet in the cold rolling. Accordingly, the N content is set to 0.002 mass % to 0.012 mass %. The N content is preferably 0.010% mass % or less in order to further reduce the blister.

S is an important element forming a MnS precipitate through reacting with Mn. The MnS precipitate mainly affects the primary recrystallization and functions to suppress the variation depending on site in grain growth in the primary recrystallization due to the hot rolling. When the Mn content is less than 0.001 mass %, the effect cannot be sufficiently obtained. On the other hand, when the Mn content is more than 0.010 mass %, the magnetic property is likely to decrease. Accordingly, the Mn content is set to 0.001 mass % to 0.010 mass %. The Mn content is preferably 0.009 mass % or less in order to further improve the magnetic property.

P increases the specific resistance of the grain-oriented electrical steel sheet to reduce the core loss. When the P content is less than 0.01 mass %, the effect cannot be sufficiently obtained. On the other hand, when the P content is more than 0.08 mass %, the cold rolling may become difficult to perform. Accordingly, the P content is set to 0.01 mass % to 0.08 mass %.

Ti forms a TiN precipitate through reacting with N. Further, Cu forms a CuS precipitate through reacting with S. These precipitates function to make the growth of the crystal grains in the finish annealing uniform irrespective of the site of the coil and suppress the variation in magnetic property of the grain-oriented electrical steel sheet. In particular, the TiN precipitate is considered to suppress the variation in grain growth in a high temperature region in the finish annealing to decrease the deviation of the magnetic property of the grain-oriented electrical steel sheet. Further, the CuS precipitate is considered to suppress the variation in grain growth in a low temperature region in the decarburization annealing and the finish annealing to decrease the deviation of the magnetic property of the grain-oriented electrical steel sheet. When the Ti content is less than 0.0020 mass % and the Cu content is less than 0.010 mass %, the effects cannot be sufficiently obtained. On the other hand, when the Ti content is more than 0.010 mass %, the TiN precipitate is excessively formed and remains even after the finish annealing. Similarly, when the Cu content is more than 0.50 mass %, the CuS precipitate is excessively formed and remains even after the finish annealing. If these precipitates remain in the grain-oriented electrical steel sheet, it is difficult to obtain a high magnetic property. Accordingly, the molten steel contains one or both of Ti and Cu in ranges of Ti: 0.010 mass % or less and Cu: 0.50 mass % or less to satisfy at least one of Ti: 0.0020 mass % or more or Cu: 0.010 mass % or more. In short, the molten steel contains at least one kind selected from a group consisting of Ti: 0.0020 mass % to 0.010 mass % and Cu: 0.010 mass % to 0.50 mass %.

Note that the lower limit of the Ti content is preferably 0.0020 mass %, and the upper limit of the Ti content is preferably 0.0080 mass %. Further, the lower limit of the Cu content is preferably 0.01 mass %, and the upper limit of the Cu content is preferably 0.10 mass %. Further, where the Ti content (mass %) is expressed as [Ti] and the Cu content (mass %) is expressed as [Cu], it is more preferable that the relation of “20×[Ti]+[Cu]≦0.18” is established and, preferably, the relation of “10×[Ti]+[Cu]≦0.07” is established.

Note that at least one kind of the following various kinds of elements may be contained in the molten steel.

Cr and Sn improve the quality of an oxide layer to be formed in the decarburization annealing and improve the quality of a glass film to be formed of the oxide layer in the finish annealing. In other words, Cr and Sn improve the magnetic property through stabilization of the formation of the oxide layer and the glass film to suppress the variation in the magnetic property. However, when the Cr content is more than 0.20 mass %, the formation of the glass film may be unstable. Further, when the Sn content is more than 0.20 mass %, the surface of the steel sheet may be less likely to be oxidized to result in insufficient formation of the glass film. Accordingly, each of the Cr content and the Sn content is preferably 0.20 mass % or less. Further, in order to sufficiently obtain the above effects, each of the Cr content and the Sn content is preferably 0.01 mass % or more. Note that Sn is a grain boundary segregation element and thus also has an effect to stabilize secondary recrystallization.

Further, the molten steel may contain Sb: 0.010 mass % to 0.20 mass %, Ni: 0.010 mass % to 0.20 mass %, Se: 0.005 mass % to 0.02 mass %, Bi: 0.005 mass % to 0.02 mass %, Pb: 0.005 mass % to 0.02 mass %, B: 0.005 mass % to 0.02 mass %, V: 0.005 mass % to 0.02 mass %, Mo: 0.005 mass % to 0.02 mass %, and/or As: 0.005 mass % to 0.02 mass %. These elements may be inhibitor strengthening elements.

In the embodiment, after the slab is produced from the molten steel with the composition, the slab is heated (Step S2). The temperature of the heating is preferably set to 1250° C. or lower from the viewpoint of energy saving.

Next, hot rolling is performed on the slab to obtain a hot-rolled steel sheet (Step S3). The thickness of the hot-rolled steel sheet is not particularly limited, and may be set to 1.8 mm to 3.5 mm.

Thereafter, annealing is performed on the hot-rolled steel sheet to obtain an annealed steel sheet (Step S4). The condition of the annealing is not particularly limited, and the annealing may be performed, for example, at a temperature of 750° C. to 1200° C. for 30 seconds to 10 minutes. The annealing improves the magnetic property.

Subsequently, cold rolling is performed on the annealed steel sheet to obtain a cold-rolled steel sheet (Step S5). The cold rolling may be performed only once or a plurality of times while an intermediate annealing is performed therebetween. The intermediate annealing is preferably performed at a temperature of 750° C. to 1200° C. for 30 seconds to 10 minutes.

Note that if the cold rolling is performed without performing the above-described intermediate annealing, it may be difficult to obtain uniform properties. On the other hand, if the cold rolling is performed a plurality of times while the intermediate annealing is performed therebetween, the uniform properties are easily obtained but the magnetic flux density may decrease. Accordingly, it is preferable to determine the number of times of the cold rolling and the presence or absence of the intermediate annealing according to the property required for and the cost of the finally obtained grain-oriented electrical steel sheet.

Further, in any case, it is preferable to set the rolling reduction at the final cold rolling to 80% to 95%.

The decarburization annealing is performed on the cold-rolled steel sheet in a wet atmosphere containing hydrogen and nitrogen at 800° C. to 950° C. after the cold rolling to obtain a decarburization annealed steel sheet (step S6). The decarburization annealing removes carbon in the steel sheet and causes primary recrystallization. When the temperature of the decarburization annealing is lower than 800° C., the crystal grain obtained through the primary recrystallization (primary recrystallization grain) is small so that the subsequent secondary recrystallization does not sufficiently appear. On the other hand, when the temperature of the decarburization annealing is higher than 950° C., the primary recrystallization grain is large so that the subsequent secondary recrystallization does not sufficiently appear.

Next, nitridation treatment is performed on the decarburization annealed steel sheet in an atmosphere containing hydrogen, nitrogen and a gas having a nitriding capability such as ammonia at 700° C. to 850° C. to obtain a nitrided steel sheet (step S7). The nitridation treatment increases the nitrogen content in the steel sheet. When the temperature of the nitridation treatment is lower than 700° C. or high than 850° C., nitrogen is difficult to be diffused to the inner part of the steel sheet so that the subsequent secondary recrystallization does not sufficiently appear.

After that, an annealing separating agent containing MgO as a main component is applied, in a water slurry, to the surface of the nitrided steel sheet, and the nitrided steel sheet is coiled. Then, batch-type finish annealing is performed on the coiled nitrided steel sheet to obtain a coiled finish-annealed steel sheet (Step S8). The finish annealing causes secondary recrystallization.

Thereafter, the coiled finish-annealed steel sheet is uncoiled, and the annealing separating agent is removed. Subsequently, a coating solution containing aluminum phosphate and colloidal silica as main components is applied to the surface of the finish-annealed steel sheet, and baking is performed thereon to form an insulating film (step S9).

In the above manner, the grain-oriented electrical steel sheet can be manufactured.

Note that the steel being an object for the hot rolling is not limited to the slab obtained through casting of the molten steel, but a so-called thin slab may be used. Further, when using the thin slab, it is not always necessary to perform the slab heating at 1250° C. or lower.

Example

Next, experiments conducted by the present inventors will be described. Conditions and so on in these experiments are examples employed to verify practicality and effects of the present invention, and the present invention is not limited to these examples.

First Experiment

First, 15 kinds of steel ingots each containing Si: 3.2 mass %, C, 0.055 mass %, Mn: 0.10 mass %, acid-soluble Al: 0.028 mass %, N: 0.003 mass %, S: 0.0060 mass %, and P: 0.030 mass %, further containing Ti and Cu in amounts listed in Table 1, and the balance composed of Fe and inevitable impurities were produced using a vacuum melting furnace. Then, annealing was performed on the steel ingots at 1150° C. for one hour, and then hot rolling was performed thereon to obtain hot-rolled steel sheets with a thickness of 2.3 mm.

Subsequently, annealing was performed on the hot-rolled steel sheets at 1100° C. for 120 seconds to obtain annealed steel sheets. Next, acid pickling was performed on the annealed steel sheets, and then cold rolling was performed on the annealed steel sheets to obtain cold-rolled steel sheets with a thickness of 0.23 mm. Subsequently, decarburization annealing was performed on the cold-rolled steel sheets in an atmosphere containing water vapor, hydrogen, and nitrogen at 860° C. for 100 seconds to obtain decarburization annealed steel sheets. Next, nitridation treatment was performed on the decarburization annealed steel sheets in an atmosphere containing hydrogen, nitrogen, and ammonia at 770° C. for 20 seconds to obtain nitrided steel sheets.

Thereafter, an annealing separating agent containing MgO as a main component was applied, in a water slurry, to the surfaces of the decarburized nitrided steel sheets. Then, finish annealing was performed on them at 1200° C. for 20 hours to obtain finish-annealed steel sheets. Subsequently, the finish-annealed steel sheets were washed with water, and then cutout into a single-plate magnetic measurement size with a width of 60 mm and a length of 300 mm. Subsequently, a coating solution containing aluminum phosphate and colloidal silica as main components was applied to the surfaces of the finish-annealed steel sheets, and baking was performed thereon to form an insulating film. In this manner, samples of the grain-oriented electrical steel sheets were obtained.

Then, the magnetic flux density B8 of each of the grain-oriented electrical steel sheets was measured. The magnetic flux density B8 is the magnetic flux density occurring in the grain-oriented electrical steel sheet when a magnetic field of 800 A/m at 50 Hz is applied thereto as described above. Note that the magnetic flux densities B8 of five single-plate samples for measurement were measured for each of the samples. Then, for each sample, the average value “average B8,” the maximum value “B8max,” and the minimum value “B8 min” were obtained. The difference “ΔB8” between the maximum value “B8max” and the minimum value “B8 min” was also obtained. The difference “AB8” is an index indicating the fluctuation range of the magnetic property. These results are listed in Table 1 together with the Ti contents and the Cu contents. Further, the evaluation results based on the average value “average B8” and the difference “ΔB8” are indicated in FIG. 1. As described above, an open circle mark in FIG. 1 indicates that the average value “average B8” was 1.90 T or more and the difference “ΔB8” was 0.030 T or less. Further, a filled circle mark in FIG. 1 indicates that the average value “average B8” was less than 1.90 T or the difference “ΔB8” was more than 0.030 T.

TABLE 1 Ti Cu AVERAGE SAMPLE CONTENT CONTENT B8 B8 max B8 min Δ B8 No. (MASS %) (MASS %) 20 × [Ti] + [Cu] 10 × [Ti] + [Cu] (T) (T) (T) (T) NOTE 1 0.0011 0.006 0.028 0.017 1.914 1.931 1.890 0.041 COMPARATIVE EXAMPLE 2 0.0024 0.007 0.055 0.031 1.913 1.923 1.902 0.021 EMBODIMENT 3 0.0048 0.006 0.102 0.054 1.911 1.919 1.897 0.022 EMBODIMENT 4 0.0084 0.008 0.176 0.092 1.902 1.915 1.889 0.026 EMBODIMENT 5 0.0109 0.005 0.223 0.114 1.877 1.887 1.866 0.021 COMPARATIVE EXAMPLE 6 0.0013 0.033 0.059 0.046 1.914 1.924 1.901 0.023 EMBODIMENT 7 0.0015 0.083 0.113 0.098 1.913 1.922 1.895 0.027 EMBODIMENT 8 0.0014 0.182 0.210 0.196 1.911 1.919 1.894 0.025 EMBODIMENT 9 0.0011 0.430 0.452 0.441 1.901 1.908 1.884 0.024 EMBODIMENT 10 0.0013 0.576 0.602 0.589 1.876 1.884 1.861 0.023 COMPARATIVE EXAMPLE 11 0.0033 0.079 0.145 0.112 1.911 1.921 1.901 0.020 EMBODIMENT 12 0.0055 0.084 0.194 0.139 1.903 1.910 1.891 0.019 EMBODIMENT 13 0.0068 0.018 0.154 0.086 1.912 1.921 1.899 0.022 EMBODIMENT 14 0.0082 0.380 0.544 0.462 1.902 1.908 1.893 0.015 EMBODIMENT 15 0.0028 0.022 0.078 0.050 1.914 1.927 1.907 0.020 EMBODIMENT

As presented in Table 1 and FIG. 1, in the samples No. 2 to No. 4, No. 6 to No. 9, and No. 11 to No. 15, in each of which the Ti content and the Cu content were within the range of the present invention, the average value “average B8” was large to be 1.90 T or more, and the difference “ΔB8” was small to be 0.030 T or less. In short, high magnetic property was obtained and the variation in magnetic property was small.

In particular, the balance between the average value “average B8” and the difference “ΔB8” was excellent in the samples No. 11, No. 13, and No. 15, in which the relation of “20×[Ti]+[Cu]≦0.18” was established where the Ti content (mass %) was expressed as [Ti] and the Cu content (mass %) was expressed as [Cu]. Among them, the balance between the average value “average B8” and the difference “ΔB8” was extremely excellent in the sample No. 15, in which the relation of “10×[Ti]+[Cu]≦0.07” was established.

On the other hand, in the sample No. 1, in which the Ti content was less than 0.0020 mass % and the Cu content was less than 0.010 mass %, the difference “ΔB8” was large to be more than 0.030 T. In short, the variation in magnetic property was large. Further, in the sample No. 5, in which the Ti content was more than 0.010 mass % and the sample No. 10, in which the Cu content was more than 0.50 mass %, a large amount of precipitate was contained to affect the finish annealing, with the result that the average value “average B8” was small to be less than 1.90 T. In short, a sufficiently high magnetic property could not be obtained.

Second Experiment

First, a steel ingot containing Si: 3.2 mass %, C, 0.051 mass %, Mn: 0.09 mass %, acid-soluble Al: 0.026 mass %, N: 0.004 mass %, S: 0.0053 mass %, P: 0.027 mass %, Ti: 0.0024 mass %, Cu: 0.029 mass %, and the balance composed of Fe and inevitable impurities was produced using a vacuum melting furnace. Then, annealing was performed on the steel ingot at 1150° C. for one hour, and then hot rolling was performed thereon to obtain a hot-rolled steel sheet with a thickness of 2.4 mm.

Subsequently, annealing was performed on the hot-rolled steel sheet at 1090° C. for 120 seconds to obtain an annealed steel sheet. Then, acid pickling was performed on the annealed steel sheet, and then cold rolling was performed on the annealed steel sheet to obtain a cold-rolled steel sheet with a thickness of 0.23 mm. Subsequently, eight pieces of steel sheets for annealing were cutout from the cold-rolled steel sheet, and decarburization annealing was performed on each of the steel sheets in an atmosphere containing water vapor, hydrogen, and nitrogen at a temperature T1 ranging from 790° C. to 960° C. listed in Table 2 for 80 seconds to obtain decarburization annealed steel sheets. Next, nitridation treatment was performed on each of the decarburization annealed steel sheets in an atmosphere containing water vapor, hydrogen, nitrogen, and ammonia at a temperature T2 ranging from 680° C. to 880° C. listed in Table 2 for 20 seconds to obtain nitrided steel sheets.

Thereafter, an annealing separating agent containing MgO as a main component was applied, in a water slurry, to the surfaces of the nitrided steel sheets. Then, finish annealing was performed on them at 1200° C. for 20 hours to obtain finish annealed steel sheets. Subsequently, treatments from the water washing to the formation of the insulating film were performed similarly to the first experiment to obtain samples of the grain-oriented electrical steel sheets.

Then, for each of the samples, the average value “average B8,” the maximum value “B8max,” the minimum value “B8 min,” and the difference “ΔB8” were obtained similarly to the first experiment. These results are listed in Table 2 together with the temperatures T1 and the temperatures T2.

TABLE 2 SAMPLE T1 T2 AVERAGE B8 B8 max B8 min Δ B8 No. (° C.) (° C.) (T) (T) (T) (T) NOTE 21 790 780 1.840 1.859 1.832 0.027 COMPARATIVE EXAMPLE 22 820 780 1.911 1.918 1.895 0.023 EMBODIMENT 23 870 780 1.924 1.931 1.908 0.023 EMBODIMENT 24 930 780 1.925 1.937 1.908 0.029 EMBODIMENT 25 860 780 1.824 1.872 1.770 0.102 COMPARATIVE EXAMPLE 26 860 670 1.896 1.907 1.880 0.027 COMPARATIVE EXAMPLE 27 860 750 1.922 1.930 1.906 0.024 EMBODIMENT 28 860 860 1.823 1.872 1.777 0.095 COMPARATIVE EXAMPLE

As presented in Table 2, in the samples No. 22 to No. 24, and No. 27, in which the temperature T1 of the decarburization annealing and the temperature T2 of the nitridation treatment were within the range of the present invention, the average value “average B8” was large to be 1.90 T or more, and the difference “ΔB8” was small to be 0.030 T or less. In short, a high magnetic property was obtained and the variation in the magnetic property was small.

On the other hand, in the sample No. 21, in which the temperature T1 of the decarburization annealing was less than 800° C., the average value “average B8” was small to be less than 1.90 T. In the sample No. 25, in which the temperature T1 of the decarburization annealing was higher than 950° C., the difference “ΔB8” was large to be over 0.030 T, and the average value “average B8” was small to be less than 1.90 T. Further, in the sample No. 26, in which the temperature T2 of the nitridation treatment was less than 700° C., the average value “average B8” was small to be less than 1.90 T. In the sample No. 28, in which the temperature T2 of the nitridation treatment is higher than 850° C., the difference “ΔB8” was large to be high than 0.030 T, and the average value “average B8” was small to be less than 1.90 T.

Third Experiment

First, 20 kinds of steel ingots each containing Si: 3.2 mass %, Mn: 0.09 mass %, acid-soluble Al: 0.026 mass %, N: 0.004 mass %, S: 0.0053 mass %, and P: 0.027 mass %, further containing C, Ti and Cu in amounts listed in Table 3, and the balance composed of Fe and inevitable impurities were produced using a vacuum melting furnace. Then, annealing was performed on the steel ingots at 1150° C. for one hour, and then hot rolling was performed to obtain hot-rolled steel sheets with a thickness of 2.4 mm.

Subsequently, annealing was performed on the hot-rolled steel sheets at 1090° C. for 120 seconds to obtain annealed steel sheets. Then, acid pickling was performed on the annealed steel sheets, and then, cold rolling was performed on the annealed steel sheets to obtain cold-rolled steel sheets with a thickness of 0.23 mm. Subsequently, steel sheets for annealing were cutout from the cold-rolled steel sheets, and decarburization annealing was performed on the steel sheets in an atmosphere containing water vapor, hydrogen, and nitrogen at 860° C. for 80 seconds to obtain decarburization annealed steel sheets. Next, nitridation treatment was performed on the decarburization annealed steel sheets in an atmosphere containing water vapor, hydrogen, nitrogen, and ammonia at 760° C. for 20 seconds to obtain nitrided steel sheets.

Thereafter, an annealing separating agent containing MgO as a main component was applied, in a water slurry, to the surfaces of the nitrided steel sheets. Then, finish annealing was performed on them at 1200° C. for 20 hours to obtain finish annealed steel sheets. Subsequently, treatments from the water washing to the formation of insulating film were performed similarly to the first experiment to obtain samples of the grain-oriented electrical steel sheets.

Then, for each of the samples, the average value “average B8,” the maximum value “B8max,” the minimum value “B8 min,” and the difference “ΔB8” were obtained similarly to the first experiment. These results are listed in Table 3 together with the C contents, the Ti contents and the Cu contents.

TABLE 3 C Ti Cu SAMPLE CONTENT CONTENT CONTENT AVERAGE B8 Δ B8 No. (MASS %) (MASS %) (MASS %) 20 × [Ti] + [Cu] 10 × [Ti] + [Cu] (T) (T) NOTE 31 0.028 0.0014 0.006 0.034 0.020 1.868 0.051 COMPARATIVE EXAMPLE 32 0.028 0.0024 0.024 0.072 0.048 1.911 0.024 EMBODIMENT 33 0.028 0.0042 0.051 0.135 0.093 1.906 0.021 EMBODIMENT 34 0.028 0.0070 0.390 0.530 0.460 1.901 0.018 EMBODIMENT 35 0.028 0.0105 0.560 0.770 0.665 1.822 0.016 COMPARATIVE EXAMPLE 36 0.039 0.0015 0.005 0.035 0.020 1.892 0.046 COMPARATIVE EXAMPLE 37 0.039 0.0023 0.022 0.068 0.045 1.913 0.022 EMBODIMENT 38 0.039 0.0044 0.053 0.141 0.097 1.907 0.019 EMBODIMENT 39 0.039 0.0075 0.350 0.500 0.425 1.902 0.017 EMBODIMENT 40 0.039 0.0110 0.590 0.810 0.700 1.843 0.015 COMPARATIVE EXAMPLE 41 0.048 0.0018 0.007 0.043 0.025 1.904 0.042 COMPARATIVE EXAMPLE 42 0.048 0.0026 0.025 0.077 0.051 1.914 0.021 EMBODIMENT 43 0.048 0.0043 0.052 0.138 0.095 1.908 0.019 EMBODIMENT 44 0.048 0.0072 0.370 0.514 0.442 1.903 0.017 EMBODIMENT 45 0.048 0.0122 0.550 0.794 0.672 1.852 0.014 COMPARATIVE EXAMPLE 46 0.057 0.0017 0.008 0.042 0.025 1.912 0.037 COMPARATIVE EXAMPLE 47 0.057 0.0024 0.026 0.074 0.050 1.915 0.020 EMBODIMENT 48 0.057 0.0046 0.054 0.146 0.100 1.908 0.017 EMBODIMENT 49 0.057 0.0074 0.380 0.528 0.454 1.907 0.015 EMBODIMENT 50 0.057 0.0109 0.540 0.758 0.649 1.871 0.013 COMPARATIVE EXAMPLE

As presented in Table 3, in the samples No. 32 to No. 34, No. 37 to No. 39, No. 42 to No. 44, and No. 47 to No. 49, in each of which the C content, the Ti content and the Cu content were within the range of the present invention, the average value “average B8” was large to be 1.90 T or more, and the difference “ΔB8” was small to be 0.025 T or less. In short, a high magnetic property was obtained and the variation in magnetic property was small. Particularly, when the C content was small, a good result was obtained.

Further, the balance between the average value “average B8” and the difference “ΔB8” was excellent in the samples No. 32, No. 33, No. 37, No. 38, No. 42, No. 43, No. 47, and No. 48, in each of which the Ti content was 0.0020 mass % to 0.080 mass %, the Cu content was 0.010 mass % to 0.10 mass %, and the relation of “20×[Ti]+[Cu]≦0.18” was established. Among them, the balance between the average value “average B8” and the difference “ΔB8” was extremely excellent in the samples No. 32, No. 37, No. 42, and No. 47, in each of which the relation of “10×[Ti]+[Cu]≦0.07” was established.

On the other hand, in the samples No. 31, No 36, No. 41, and No. 46, in each of which the Ti content was less than 0.020 mass %, and the Cu content was less than 0.010 mass %, the difference “ΔB8” was large to be higher than 0.030 T. Among them, in the samples No. 31 and No. 36, in each of which the C content was low, the average value “average B8” was further small to be less than 1.90 T. Further, in the samples No. 35, No. 40, No. 45, and No. 50, in each of which the Ti content was higher than 0.010 mass %, and the Cu content was higher than 0.50 mass %, the average value “average B8” was small to be less than 1.90 T.

Fourth Experiment

First, 10 kinds of steel ingots each containing Si: 3.2 mass %, C, 0.048 mass %, Mn: 0.08 mass %, acid-soluble Al: 0.028 mass %, N: 0.004 mass %, S: 0.0061 mass %, P: 0.033 mass %, Ti: 0.0024 mass %, and Cu: 0.029 mass %, further containing Cr and Sn in amounts listed in Table 4, and the balance composed of Fe and inevitable impurities were produced using a vacuum melting furnace. Then, annealing was performed on the steel ingots at 1100° C. for one hour, and then hot rolling was performed thereon to obtain hot-rolled steel sheets with a thickness of 2.3 mm.

Subsequently, annealing was performed on the hot-rolled steel sheets at 1080° C. for 120 seconds to obtain annealed steel sheets. Then, acid pickling was performed on the annealed steel sheets, and then cold rolling was performed on the annealed steel sheets to obtain cold-rolled steel sheets with a thickness of 0.23 mm. Subsequently, decarburization annealing was performed on the cold-rolled steel sheets in an atmosphere containing water vapor, hydrogen, and nitrogen at 870° C. for 90 seconds to obtain decarburization annealed steel sheets. Next, nitridation treatment was performed on the decarburization annealed steel sheets in an atmosphere containing hydrogen, nitrogen, and ammonia at 760° C. for 20 seconds to obtain nitrided steel sheets.

Thereafter, an annealing separating agent containing MgO as a main component was applied, in a water slurry, to the surfaces of the nitrided steel sheets. Then, finish annealing was performed on them at 1200° C. for 20 hours to obtain finish annealed steel sheets. Subsequently, treatments from the water washing to the formation of the insulating film were performed similarly to the first experiment to obtain samples of the grain-oriented electrical steel sheets.

Then, for each of the samples, the average value “average B8,” the maximum value “B8max,” the minimum value “B8 min,” and the difference “ΔB8” were obtained similarly to the first experiment. These results are listed in Table 4 together with the Cr contents and the Sn contents.

TABLE 4 Cr Sn Content Content B8 B8 Sample (mass (mass Average max min ΔB8 No. %) %) B8 (T) (T) (T) (T) NOTE 51 0.007 0.008 1.904 1.918 1.890 0.028 Embodiment 52 0.068 0.003 1.915 1.927 1.903 0.024 Embodiment 53 0.141 0.008 1.915 1.928 1.904 0.024 Embodiment 54 0.214 0.006 1.903 1.917 1.888 0.029 Embodiment 55 0.003 0.042 1.917 1.928 1.906 0.022 Embodiment 56 0.006 0.087 1.918 1.928 1.907 0.021 Embodiment 57 0.005 0.264 1.902 1.915 1.888 0.027 Embodiment 58 0.070 0.124 1.913 1.922 1.898 0.024 Embodiment 59 0.150 0.035 1.914 1.924 1.901 0.023 Embodiment 60 0.170 0.154 1.912 1.922 1.897 0.025 Embodiment

As presented in Table 4, in any of the samples Nos. 51 to 60, the average value “average B8” was large to be 1.90 T or more and the difference “ΔB8” was small to be 0.030 T or less. In short, a high magnetic property was obtained and the variation in the magnetic property was small. Among them, in the samples No. 52, No. 53, No. 55, No. 56, and No. 58 to No. 60, each of which contains 0.010 mass % to 0.20 mass % of Cr and/or 0.010 mass % to 0.20 mass % of Sn, the average value “average B8” was particularly large to be 1.91 T or more, and the difference “ΔB8” was particularly small to be 0.025 T or less.

INDUSTRIAL APPLICABILITY

The present invention is applicable, for example, in electrical steel sheet manufacturing industries and electrical steel sheet using industries. 

1. A method of manufacturing a grain-oriented electrical steel sheet, comprising: performing hot rolling of a steel containing Si: 2.5 mass % to 4.0 mass %, C, 0.01 mass % to 0.060 mass %, Mn: 0.05 mass % to 0.20 mass %, acid-soluble Al: 0.020 mass % to 0.040 mass %, N: 0.002 mass % to 0.012 mass %, S: 0.001 mass % to 0.010 mass %, and P: 0.01 mass % to 0.08 mass %, further containing at least one kind selected from a group consisting of Ti: 0.0020 mass % to 0.010 mass % and Cu: 0.010 mass % to 0.50 mass %, and a balance composed of Fe and inevitable impurities, to obtain a hot-rolled steel sheet; performing annealing on the hot-rolled steel sheet to obtain an annealed steel sheet; performing cold rolling on the annealed steel sheet to obtain a cold-rolled steel sheet; performing decarburization annealing on the cold-rolled steel sheet at a temperature of 800° C. to 950° C. to obtain a decarburization annealed steel sheet; then, performing nitridation treatment on the decarburization annealed steel sheet at 700° C. to 850° C. to obtain a nitrided steel sheet; and performing finish annealing on the nitrided steel sheet.
 2. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the hot rolling on the steel is performed after heating the steel to a temperature of 1250° C. or lower.
 3. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the steel further contains at least one kind selected from a group consisting of Cr: 0.010 mass % to 0.20 mass %, Sn: 0.010 mass % to 0.20 mass %, Sb: 0.010 mass % to 0.20 mass %, Ni: 0.010 mass % to 0.20 mass %, Se: 0.005 mass % to 0.02 mass %, Bi: 0.005 mass % to 0.02 mass %, Pb: 0.005 mass % to 0.02 mass %, B: 0.005 mass % to 0.02 mass %, V: 0.005 mass % to 0.02 mass %, Mo: 0.005 mass % to 0.02 mass %, and As: 0.005 mass % to 0.02 mass %.
 4. The method of manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein the steel further contains at least one kind selected from a group consisting of Cr: 0.010 mass % to 0.20 mass %, Sn: 0.010 mass % to 0.20 mass %, Sb: 0.010 mass % to 0.20 mass %, Ni: 0.010 mass % to 0.20 mass %, Se: 0.005 mass % to 0.02 mass %, Bi: 0.005 mass % to 0.02 mass %, Pb: 0.005 mass % to 0.02 mass %, B: 0.005 mass % to 0.02 mass %, V: 0.005 mass % to 0.02 mass %, Mo: 0.005 mass % to 0.02 mass %, and As: 0.005 mass % to 0.02 mass %.
 5. The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein a Ti content in the steel is 0.0020 mass % to 0.0080 mass %, a Cu content in the steel is 0.01 mass % to 0.10 mass %, and a relation of “20×[Ti]+[Cu]≦0.18” is established where the Ti content (mass %) in the steel is expressed as [Ti] and the Cu content (mass %) is expressed as [Cu].
 6. The method of manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein a Ti content in the steel is 0.0020 mass % to 0.0080 mass %, a Cu content in the steel is 0.01 mass % to 0.10 mass %, and a relation of “20×[Ti]+[Cu]≦0.18” is established where the Ti content (mass %) in the steel is expressed as [Ti] and the Cu content (mass %) is expressed as [Cu].
 7. The method of manufacturing a grain-oriented electrical steel sheet according to claim 3, wherein a Ti content in the steel is 0.0020 mass % to 0.0080 mass %, a Cu content in the steel is 0.01 mass % to 0.10 mass %, and a relation of “20×[Ti]+[Cu]≦0.18” is established where the Ti content (mass %) in the steel is expressed as [Ti] and the Cu content (mass %) is expressed as [Cu].
 8. The method of manufacturing a grain-oriented electrical steel sheet according to claim 4, wherein a Ti content in the steel is 0.0020 mass % to 0.0080 mass %, a Cu content in the steel is 0.01 mass % to 0.10 mass %, and a relation of “20×[Ti]+[Cu]≦0.18” is established where the Ti content (mass %) in the steel is expressed as [Ti] and the Cu content (mass %) is expressed as [Cu].
 9. The method of manufacturing a grain-oriented electrical steel sheet according wherein a relation of “10×[Ti]+[Cu]≦0.07” is established to claim
 5. 10. The method of manufacturing a grain-oriented electrical steel sheet according to claim 6, wherein a relation of “10×[Ti]+[Cu]≦0.07” is established.
 11. The method of manufacturing a grain-oriented electrical steel sheet according to claim 7, wherein a relation of “10×[Ti]+[Cu]≦0.07” is established.
 12. The method of manufacturing a grain-oriented electrical steel sheet according to claim 8, wherein a relation of “10×[Ti]+[Cu]≦0.07” is established. 